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Abstract:
Three-dimensional lattice Boltzmann (LB) simulations were carried out to investigate the mass transport and reaction inside and around catalytic porous particles used in uidized beds for the synthesis of methane from biogas (H2/CO mixtures). The 3D internal porous structure of a real particle with a size of about 200 microns was assessed with Synchrotron-based X-ray tomography (XTM). Pore-resolved simulations with a suitable LB model for catalytic reactions in microows revealed that the preferential diffusion of H2 and the resulting inability of CO to penetrate deep inside the pores led to segregated distributions of the two species, hindering reaction in the downstream parts of the particle and rendering CO the de cient reactant. The implications for practical uidized beds are that, for specific bed zones with a high dense-phase (solid-phase) volume fraction and a predominant ow direction, CO starvation may occur and may thus limit the methane yields. Subsequent parametric studies of spherical particles with a diameter 2R = 150 microns and an artificially generated porosity allowed comparisons of LB results with radially symmetric analytical continuum-model solutions. The LB simulations revealed appreciable angular variations in the CO concentration at the external surface (r = R) and the outer layers of particle, especially at the higher investigated Damkoehler numbers, which were however diminished as the core of the particle (r---> 0) was approached. The large angular variations, in conjunction with the skewed CO distributions towards their lower values, led to LB-computed particle effectiveness factors 30% lower than the corresponding analytical solutions.
Laboratory for Simulation and Modelling